add highlighting support for BUG, FIXME, NOTE and TODO keywords in comments

This pull request adds some special highlighting for the following keywords:
`BUG:`, `FIXME:`, `NOTE:` and `TODO:`

before commit:
![todo1](https://f.cloud.github.com/assets/1828443/1328968/03856468-350e-11e3-8b17-5813c0e67127.png)

after commit:
![todo](https://f.cloud.github.com/assets/1828443/1328969/0751b4d4-350e-11e3-9d00-9f8c4f456da7.png)